The Moon Illusion

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This is known as the Ponzo illusion , discovered by Italian psychologist Mario Ponzo in In a real moonrise, it's thought that distant trees, buildings and landscape features play the role of converging lines. We're built to think that objects near the horizon are usually more distant than those overhead because they appear to lie behind and beyond foreground objects. The Moon's apparent size relates to our perception of the sky as a flattened dome.

The 'Big Moon' Illusion May Be All in Your Head

When the Moon is viewed near the horizon, our brains judge it to be farther away than when seen near the zenith. Based on that assumption, we inflate its size to accommodate the false extra distance. Click graphic for more information. But for extraterrestrial bodies such as the Moon, Sun and star groupings, which are identical in size whether on the horizon or at the zenith, we have no reference. Therefore, when we gaze at a horizon Moon, which clearly lies beyond every object in the foreground, our brains assume it must be farther away than the overhead version.

We compensate for this perception by inflating the Moon's size.

In a sense, our brains force the Moon to meet our expectations of how big it should be. The Moon looms huge in this telephoto view taken of moonrise over the Swedish village Marieby in June The "relative size" theory is yet another explanation for the difference in the Moon's apparent size, as illustrated here by the Ebbinghaus illusion. The lower central circle surrounded by small circles represents the horizon Moon with foreground objects like trees and buildings, while the upper central circle represents the zenith moon surrounded by large expanses of sky.

To many, the bottom circle looks larger, but they're both the same size. Think of all the rectangular and square objects we come across during the day. Unless you're staring square-on at these shapes, they should look like trapezoids of all dimensions.

Confirm on Your Own

Do they? What's funny in all of this is that the rising Moon is actually 1. With the Moon near the zenith, we look straight into space with no Earth in the way. Take a look for yourself the next few nights when the frosty Moon climbs above the eastern horizon. Do both perspectives swell the Moon's apparent size equally? Does it look obviously smaller?

Moon illusion - Wikipedia

A magnificent rising Moon frames Lick Observatory atop Mt. Hamilton near San Jose, Calif. Rick Baldridge. It occurs around a. Talk about illusions. You'll be watching the puny 2,mile-diameter Moon cover a star nearly 18, times its size. Need a map to go with that Moon and potential Moon illusion? From my home in San Francisco, the eastern horizon is the ridge of Potrero Hill only about a mile distant. But if I walk up to the top of Bernal Hill, I can see all the way across the bay to the east bay hills, about 20 miles away. The Moon rising over the east bay hills looks huge!

I have sometimes tried to figure out whether or not it looks larger near the horizon, but it never really does. I have no idea what that means. It was taken with a very wide angle lens. If you ever have seen the old moon in the arms of the new moon, the crescent moon appears much larger than the new moon which is made visible by dim Earth-shine.

The elevated moon usually appears smaller than the horizon moon of equal angular size. This is the moon illusion. Distance cues may enable the perceptual system to place the horizon moon at an effectively greater distance than the elevated moon, thus making it appear as larger. This explanation is related to the size-distance invariance hypothesis.

However, the larger horizon moon is usually judged as closer than the smaller zenith moon. A bias to expect an apparently large object to be closer than a smaller object may account for this conflict.